1. Field of the Invention
The present invention relates to improved two stage combustion of sulfur and nitrogen containing carbonaceous materials, such as coal, low grade fuels and wastes, sludges and liquids. The first stage is carried out in a fluidized bed with density/size selective solids withdrawal and the second stage in a cyclonic combustor. More specifically, this invention relates to reduction of sulfur and nitrogen oxides in combustion flue gases. In situ conversion of harmful and/or unstable sulfur compounds formed during first stage combustion yields an environmentally acceptable sulfur compound within the fluidized bed and at least one selective solids withdrawal conduit. Conversion of fuel-bound nitrogen to molecular nitrogen within the fluidized bed results in low nitrogen oxides in combustion flue gases. The combustion process of this invention may be used in the combustion of a wide variety of carbonaceous materials and in processes for production of thermal energy, such as boilers, for steam production. This invention provides removal of undesired gaseous sulfur compounds formed during combustion by their conversion in the fluidized bed and solids withdrawal conduit to an environmentally acceptable solid form which are safely discharged with agglomerated non-leachable ash.
2. Description of the Prior Art
Carbonaceous materials such as coal may be burned by contacting with oxygen containing gas or may be gasified by contacting with a steam and oxygen containing gas, such as air, at elevated temperatures generally in the range of about 1400.degree. to 3400.degree. F. and at atmospheric and elevated pressures. When using sulfur and nitrogen containing carbonaceous materials, products of the combustion include sulfur and nitrogen oxides and products of the gasification reaction include sulfur compounds such as hydrogen sulfide and carbonyl sulfide which are environmentally undesirable.
The U-Gas Process, as disclosed in Oil and Gas Journal--Aug. 1, 1977, p. 51 et seq., and in U.S. Pat. No. 4,315,758, the teachings of which are incorporated herein by reference, overcomes problems associated with the agglomeration of ash particles within the fluidized bed gasification zone as taught by patents cited therein, but requires the removal of toxic sulfur compounds from the product gas stream. Sulfur compounds present in the carbonaceous feed material are typically converted primarily to gaseous hydrogen sulfide and carbonyl sulfide during gasification in a fluidized bed reactor. These sulfur compounds must be removed from the product gas stream before it is utilized as fuel gas, or for SNG production, ammonia synthesis, and the like to prevent the formation of toxic sulfur oxides or to avoid poisoning synthesis catalysts. One conventional method for removing sulfur compounds from coal gasification product gas requires cooling of the product gas to approximately ambient temperatures and scrubbing it with a chemical or physical absorbent. This method is undesirable since it results in a significant loss in process efficiency, it requires extensive equipment, and it generally produces a considerable amount of waste materials.
U.S. Pat. Nos. 4,457,289 and 4,469,050 teach gasification and combustion in a circulating fluidized bed system with adiabatic or non-adiabatic combustion with an upper cyclonic second stage. More than 50 percent of the total combustion air is fed tangentially to the cyclonic region providing a swirl number greater than 0.6 and a Reynolds number greater than 18,000. The '289 and '050 patents do not teach reduction of SO.sub.x and NO.sub.x in the combustor effluent, nor do they suggest removal of sulfur compounds in an environmentally acceptable solid form.
Circulating fluidized bed combustors are known using inert fluidizing media with addition of limestone to reduce SO.sub.x emission from combustion of high sulfur coal. In such circulating fluidized bed combustor operation the coal ash is elutriated with the gas and exits the top of the vessel, necessitating the utilization of. mechanical removal equipment, such as cyclones, baghouses and the like. In using two stage combustion and limiting operating temperatures for NO.sub.x reduction to about 1500.degree. F., the circulating fluidized bed combustor results in large equipment size.
It is known to remove sulfur oxides from flue gases produced by the combustion of coal by contacting the flue gases with alkaline earth metal oxides. Finely divided materials, such as limestone or dolomite may be injected directly into a combustion furnace at a point somewhat removed from the flame, or particulate limestone or dolomite may be used in a fixed, moving or fluidized bed to contact and absorb sulfur oxide gases contained in a flue gas stream. It is also known that combustion of sulfur bearing coal or oil may be conducted in a fluidized bed of limestone which reacts with sulfur oxide gases produced during the combustion. Coal combustion reaction conditions differ significantly from coal or carbonaceous solids gasification reaction conditions, however, since in single stage combustion reactors an excess of oxygen containing gas is required to promote complete combustion, while carbonaceous solids gasification is carried out under reducing conditions. U.S. Pat. No. 3,949,684 teaches total oxidation of sulfur containing coal in a fluidized bed with conversion of sulfur to a metallic sulfate compound. Since oxidizing conditions are present in a single stage combustion reactor, sulfur compounds present are converted to sulfur dioxide gas which reacts with limestone to produce calcium sulfate, a stable solid material which does not require further processing prior to discharge and disposal. However, two stage combustors with sub-stoichiometric combustion in a first combustion zone provide reducing conditions similar to a gasifier.
Introduction of alkaline earth metal oxides, such as calcined limestone or calcined dolomite with the coal feed into a coal gasifier having a flat distributor plate is taught by U.S. Pat. No. 3,969,089 and the patents cited therein to reduce the presence of sulfur compounds in coal gasification product gas. The hydrogen sulfide and carbonyl sulfide gas compounds formed during coal gasification react with the calcium oxide to produce solid calcium sulfide. Calcium sulfide is, however, unstable and decomposes under atmospheric conditions in the presence of water to produce hydrogen sulfide, a poisonous gas. The sulfurized calcined limestone is regenerated in a separate regenerator operated under oxidizing conditions to form sulfur dioxide gas and calcium oxide. This additional process step results in a loss of process efficiency, since it requires a separate reactor and transport of hot solids in water-free condition from one vessel to another and still results in sulfur oxides which are not desired to be released to the atmosphere. U.S. Pat. No. 3,847,563 teaches gasification of coal in a dual fluidized bed wherein desulfurization takes place in a first fluidized bed by addition of calcium oxide to the top of the solids bed from which it is removed as calcium sulfide. U.S. Pat. No. 3,970,434 teaches gasification of coal in admixture with alkaline earth metal oxides for absorption of sulfur and later combustion of the alkaline earth sulfide in a separate unit. U.S. Pat. No. 3,977,844 teaches gasification of coal in the presence of an oxide, hydroxide, bicarbonate or carbonate of an alkali or alkaline earth metal forming a metal sulfide which is transferred to a separator reactor for conversion to gaseous hydrogen sulfide which may be utilized in Claus process technology.